Article pubs.acs.org/IECR
Multivalent Cu-Doped ZnO Nanoparticles with Full Solar Spectrum Absorbance and Enhanced Photoactivity Niya Mary Jacob,† Giridhar Madras,‡ Nagaraju Kottam,§ and Tiju Thomas*,† †
Materials Research Centre and ‡Department of Chemical Engineering, Indian Institute of Science, Bangalore-560012, Karnataka, India § Department of Chemistry, M.S. Ramaiah Institute of Technology, Bangalore-560012, Karnataka, India S Supporting Information *
ABSTRACT: Full solar spectrum absorbers are widely pursued for applications related to photocatalysis and photovoltaics. Here we report multivalent Cu-doped ZnO nanoparticles which exhibit full solar spectrum absorbance and high photoactivity. Metathesis-based, green-chemical approaches with synthesis yield of ∼100% are used. Cu incorporation in ZnO results in an increase of average solar spectrum absorbance from a mere 0.4% to 34%. On the other hand, (Zn, Cu)O composites result in materials with up to 64% average solar spectrum absorbance. Doped systems operate well under both visible and UV illumination. The nanomaterials prepared are characterized by using X-ray powder diffraction (XRD), scanning electron microscopy (SEM), Brunauer−Emmett−Teller (BET) surface area analysis, and X-ray photoelectron spectroscopy (XPS). Photocatalysts explored have particle sizes ≥50 nm. This is deliberately done in order to avoid the nanotoxic size regime of ZnO. Despite the large particle size and low specific surface area (0.5 M) performed better than those synthesized using lower concentration precursor solutions ( 60 m2·g−1). This is because our samples have particle sizes >50 nm. In fact we prefer this size regime, since we wish to avoid the nanotoxic regime for oxides. We believe that this, coupled with the fact that processes presented are green, soft, and high yield, makes our resulting photocatalyst attractive for practical applications. However we note one limitation, in this context. One of the factors determining the commercial viability of a photocatalyst is its recyclability. It is important to point to the fact that, despite its promise, ZnO has lower recyclability when compared to TiO2. This is because the chemical stability of ZnO is less than that of TiO2; furthermore, ZnO surfaces are known to be relatively pH sensitive.48 This is reflected in our studies wherein we see catalyst inactivation soon after three runs. Efforts are underway to improve the surface chemical stability of ZnO. Also we are currently working on regenerating the ZnO surfaces using water-free, purely physical processes. Finally we would also like to point out that even dilute, but careful, doping of Cu in ZnO (using the appropriate soft synthesis conditions) can significantly enhance the visible light absorbance, as is seen in Figure 6b. However when the same doping is done using other conventional approaches, say the hydrothermal method, significant improvement in the visible light absorbance is not observed.46 Hence soft and green chemistry is useful for scalable synthesis and modification of viable photocatalysts.
light). Extrapolation of the linear part of the plot is used to extract Eg (Figure 6c). Eg values obtained are confirmed independently using the KM method. In this method (1 − R∞)2/2R∞ (where R∞ = reflectance of a thick layer of sample material) is plotted with respect to λ (Figure 6d). The thickness of the sample material should be several times the wavelength of the incident light, for the KM method to work accurately. For all practical purposes, a 1 mm thick film satisfies this condition for the wavelengths chosen (i.e., 380−1000 nm). The point where the KM function has the highest positive slope is used to determine Eg. We note that the band gap of as-synthesized ZnO (2.96− 3.14 eV) is at the lower end of the usually reported values for ZnO, which tend to vary between 2.98 and 3.4 eV.43,44 It is important to note that ultrapure ZnO made using methods such as metal organic chemical vapor deposition (MOCVD) always tend to have higher Eg values (typically >3.2 eV).45 In contrast, ZnO synthesized using methods wherein there is a significant possibility of unintentional doping (e.g., hydrothermal or solvothermal synthesis), one almost always obtains lower values of Eg (
